This invention relates to analog conversion circuits and, more particularly, to an analog circuit functioning as a read-only memory for converting an analog or digital input to an output which is functionally related to the antilog of the input.
Instrumentation and control equipment uses a variety of circuitry for producing output wave forms having various characteristics. For example, square waves, triangular waves, and sinusoidal waves are common wave forms generated by function generators. These wave forms may be obtained from either digital or analog-type systems.
Yet another function which may typically be used is a logarithmic or, conversely, an anti-logarithmic wave form. Such circuits might be used to obtain a logarithmic relationship between an input and an output. One use for such a circuit would be to obtain an improved zero point resolution with respect to a scale having an output over several orders of magnitude. Still other uses would be equivalent to an analog read only memory for directly converting an input to its antilogarithmic equivalent.
Logarithmic and anti-logarithmic conversion circuits exist in the prior art. Digital circuits typically use a read-only memory for converting an input to its logarithm or anti-logarithm. Analog circuits are also available. In one conversion scheme, the forward conduction characteristics of a diode generally approximate an exponential relationship between input voltage and current. Thus, a logarithmic relationship may be obtained between the input and output by converting the output current to a voltage for processing over the appropriate forward conduction range of the diode. In yet another embodiment, diodes are used as shunting devices to approximate a logarithmic output by a stepped removal of resistors to produce an output logarithmically related to the input.
Prior art devices having a high degree of accuracy and resolution are, however, expensive. Further, digital techniques, while accurate, require analog-to-digital conversion circuits at the input and digital-to-analog conversion circuits at the output for use in an analog control scheme. Circuits using diode forward conduction characteristics may be relatively simple but are relatively inaccurate due to the temperature sensitivity of the characteristics. Temperature compensating components may be used, but only at increased complexity and cost. Diode shunting circuitry forms only an approximate waveform and a large number of resistors and diodes are required to obtain an accurate resolution, particularly about the zero point. These and other disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for obtaining a logarithmic relationship between an input signal and a corresponding output signal.